Origin of the southern Ryukyu Forearc Terrace: A seaward-protrusion of arc crust elevated by subducted asperities?

Abstract

• The southern Ryukyu Forearc Terrace exhibits an extremely high free-air gravity anomaly and a thick upper crust. • The rest of the southern Ryukyu forearc region is characterized by prominent forearc basins and a large-volume accretionary prism. • The forearc terrace represents a remnant seaward-protruded arc crust. • The bulged and uplifted basement of the forearc terrace is most probably caused by the subduction of asperities. A forearc terrace with distinct topography and gravity anomalies lies within the Ryukyu forearc between 125°E and 127°E. The origin and tectonic affinity of this terrace remain uncertain. This study attempts to depict the structural characteristics of the forearc terrace and to trace its origin and formation mechanisms through a comprehensive examination of newly acquired geophysical data combined with the results of previous surveys. The forearc terrace exhibits shallow bathymetry, an extremely high free-air gravity anomaly, a shallow buried acoustic basement, a thin sedimentary cover, and a thick upper crust. In contrast, the rest of the southern Ryukyu forearc region is characterized by a steep upper forearc slope, prominent forearc basins, a deeply buried acoustic basement, and a large-volume accretionary prism. The forearc terrace represents a remnant seaward-protrusion of arc crust created after the frontal portion of the arc crust had been removed by subducted ridges or other types of linear bathymetric highs. This is in contrast to other formation mechanisms such as a block sliding seaward along strike-slip faults or an accretion of exotic terrane from the outer margin of the original arc crust after being scraped off from the subducting oceanic crust. The bulged and uplifted basement of the forearc terrace is most likely associated with subducted seamounts. Further studies, especially deep-sea drilling, are necessary to verify the nature and origin of the forearc terrace.